Presently video information is generated and received as an analog signal of considerable bandwidth and dynamic range, with transmission primarily one way. On the other hand, voice and data information are often transmitted bi-directionally and are digital in form. They usually require much less bandwidth and dynamic range than do multi-channel video signals. This dichotomy has made purely electronic multiplexing of wideband analog video with narrowband voice and digital data signals inherently complex. But, since the preferred method for telephone transmission is digital, once the telephone network service embodies video transmission as part of its offering then video transmission very likely will also be preferred digital.
Several architectures utilize optical wavelength division multiplexing (WDM) to avoid problems of electrically multiplexing wideband analog video with narrowband digital signals. This approach suffers the disadvantage of requiring multiple optical sources and detectors per subscriber, as well as requiring specialized optical coupling components.
Many architectures use analog video transmission through single mode fiber, directly modulating the laser with AM or FM techniques for transmission of several channels of video information to the subscriber. These systems require precise linearity and stabilization of the laser source, and with few exceptions are not very compatible with low cost lasers.
Two conventional schemes exist for multiplexing and digitally transmitting multichannel information over optical fiber networks. The first involves individually digitizing the input channels by assigning an analog-to-digital (A/D) converter to each channel with the resulting digital tributary data being time division multiplexed (TDM) into a high speed digital bit stream. This allows the parameters of each A/D converter to be tailored to the exact requirements of each channel. Thus, the least number of bits can be transmitted for the amount of information present, yielding the maximum bandwidth efficiency. In a second approach, a single A/D converter is timeshared among several analog channels. Analog switches select the appropriate channel, each in turn being digitized. It is difficult to vary the dynamic range and bandwidth resources in the digitizer to efficiently accommodate the requirements of all channels being digitized. Thus, the A/D sample rate and dynamic range must be set according to the worst case channel.